Building construction method and system

ABSTRACT

A building system that includes a plurality of screw piles ( 34, 36 ) positioned in the ground and one or more perimeter beams ( 16 ), wherein at least part of the perimeter beam ( 16 ) has a sloped lower surface ( 66, 68 ). The sloped lower surface of the perimeter beams ( 16 ) can divert swelling soils away from the perimeter beam ( 16 ). The swelling soils can be diverted into voids.

FIELD OF THE INVENTION

The present invention relates to a method for constructing a building slab. In another aspect, the present invention relates to a building slab system.

BACKGROUND TO THE INVENTION

Construction of buildings, such as residential housing and commercial buildings, on building sites that have reactive soils or problems soils, has posed many challenges to the construction industry. As is known to persons skilled in the art, reactive soils undergo significant swelling as their moisture content increases and significant shrinkage as their moisture content decreases. Consequently, reactive soils typically exhibit significant variations in soil height. Therefore, the construction industry faces significant issues in designing and building foundations and slabs for buildings located on reactive soil sites.

A number of building sites are now becoming available that have soils that are classified as “problem soils”. Such building sites typically have large quantities of fill placed on those sites. This can cause difficulty because the long-term behaviour of the fill is often unknown. For example, if the building site has had relatively recent additions of fill, the fill may undergo significant settlement as time passes.

A number of possible solutions have been tried to combat the difficulties faced in building on building sites that have reactive soils or problems soils. These include:

a) construction of a suspended slab. In this construction technique, piers are formed by boring holes into the ground until a depth greater than the “zone of influence” with reactive clay soils, or beyond the zone of potential settlement of low load bearing soils or fill (problem soils) is reached. The “zone of influence” is the region of reactive soils. Effectively, the pier depth should extend down to a depth where the ground structure is relatively stable. The pier holes then have appropriate reinforcement material placed in them and are filled with concrete. When the concrete sets, strong concrete piers are formed in the ground, with the bottom of those piers resting in a stable soil zone. A very strong slab is then constructed. This slab rests on the piers. If the soil shrinks or settles, the slab is supported by the piers.

This solution is suitable for use in soils that have the potential to settle. However, it is not appropriate for use in reactive clay soils as swelling of the soil can cause the slab to flex or twist under the hydraulic forces exerted by the swelling clay soils, which leads to cracking in the concrete slab and lifting of the concrete slab with the concrete piers. Further, this slab construction method can be quite expensive as the concrete piers normally have to be of great depth. A larger ratio of bored piers are also used as engineers space those piers quite closely to overcome deleterious effects arising from inconsistency in quality in the piers and the accumulation of debris in the bored holes prior to concreting of the piers. It will be understood that debris underneath the concrete pier forms a compressible layer under the pier, thereby adversely affecting its load capacity.

b) as a variation of the system discussed in (a) above, screw piles are used instead of concrete piles. Screw piles typically comprise a long steel shaft having a screw flight located towards the lower end thereof. In this construction method, the screw piles are screwed into the ground and the slab is built and suspended on the screw piles. The piles are connected to the slab or keyed into the slab. Again, this solution is useful in settling soils but has limitations in reactive soils because the slab can be lifted off the screw piers. A traditional suspended slab is not designed to cope with lifting loads applied by a swelling reactive clay soil. These slabs are designed to handle tensile forces applied to the bottom of the slab (by virtue of placement of steel reinforcing at the bottom of the slab) but the top part of the slab has only poor resistance to tensile forces. Thus, lifting forces applied by a swelling soil tend to cause cracking in such slabs.

In order to address these issues, screw piling suppliers, contractors and their associated engineers have taken the step of ripping or scarifying the top part of the soil prior to forming the slab. This loosens the top part of the soil and, in some instances, soil is removed from under the position of the slab in an attempt to create an aerated cushion of soil to mitigate the pressure applied by swelling soils. This can have the unfortunate consequence of causing water to pool under the slab at the base of the ripped or scarified area, thereby concentrating water at a lower depth in the soil. Effectively, the ripping process has the potential to move the zone of influence deeper into the undisturbed ground and the scarifying or ripping process can cause the zone of influence to move to a level below the screw piles. This is very undesirable.

c) on building sites where reactive soils are present, floating concrete slabs are frequently employed, particularly in residential housing and small commercial buildings. Floating concrete slabs are formed on the soil surface. The floating slab is designed to be very stiff and very strong. If the soil shrinks (for example, during a prolonged dry spell), the soil shrinks away from the concrete slab, particularly around the edges of the slab. These slabs are designed to be strong enough so that any unsupported regions underneath the slab do not break. Similarly, if the soil expands, the slab can shift upwardly with the expanding soil. Such slabs are known as heavy duty grillage rafts and are exceedingly strong. However, they require very large amounts of concrete and reinforcing steel and require significant site preparation to complete. As a result are very expensive to construct.

In order to address these issues, so-called “waffle raft” slabs were developed. Waffle raft slabs are formed by setting out formwork on the ground, positioning a plurality of void forming elements (typically polystyrene boxes or, on occasions, cardboard boxes) at desired positions, placing appropriate reinforcement material and pouring concrete. The void forming elements reduce the amount of concrete required in the slab and result in the formation of a slab having a plurality of ribs, set out typically in a grid pattern. If the slab could be viewed from underneath, it would resemble the surface of a waffle, hence the name “waffle raft slab”.

Waffle raft slabs have been widely used in building on reactive soil sites. However, to account for the maximum possible swelling and shrinkage potential of the soil, waffle raft slabs have to be engineered with great strength. This, of course, increases the cost of the slab. Furthermore, waffle raft slabs do not perform as well as suspended slabs on sites where differential settling is possible. Waffle raft slabs also suffer from a problem known as hogging, which occurs if the soil shrinks away from the edge beam of the slab. If this occurs, the load carried by the edge beam of the slab is not transferred into the ground below the edge beams but rather is transferred to the interior ribs of the slab. These ribs are not designed to carry the load of the structure and they will flex and bow under that load, causing structural failure.

In our Australian patent application number 2007237161, the entire contents of which are incorporated herein by cross reference, we describe a building slab system comprising a plurality of screw piles positioned in the ground and a concrete slab constructed on and formed above the screw piles, characterised in that one or more of the screw piles are fitted with pile caps having an upper surface that extends beyond an outer periphery of an upper part of the screw piles. The pile caps form a structural slip joint between the slab and the screw piles. If a reactive soil underneath the building slab swells, the slab can move upwardly with the swelling soil because the pile caps can slide upwardly along the shaft of the screw piles. If the soil underneath the building slab shrinks, the pile caps move downwardly until they rest on the top of the screw piles. The slab is then supported by the screw piles. This building slab system and building method provides a number of advantages over prior art systems.

Other building systems utilise a plurality of beams that are supported on piers or piles. The beams, for example, may comprise a plurality of perimeter beams. The perimeter beams are typically made from reinforced concrete. The perimeter beams may be joined to each other, such as by use of bolts or the like, to form a strong foundation or base for the building. The perimeter beams may be positioned above the ground, may rest on the ground or may be placed in trenches dug into the ground. The building is subsequently constructed on the perimeter beams. Advantageously, the perimeter beams may be precast in a factory and transported to the building site. This is useful in that quality control of the perimeter beams can be assured in the factory and the quality of the foundation of the building is not so reliant upon a relatively skilled labour force installing or constructing the foundations for the building.

BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a building system, characterised in that the building system includes a plurality of screw piles positioned in the ground and one or more perimeter beams, wherein at least part of the perimeter beam has a sloped lower surface.

Desirably, the sloped lower surface of the perimeter beam diverts swelling soil away from the perimeter beam.

In some embodiments, the sloped lower surface of the perimeter beam comprises two sloped lower surfaces. For example, the beams may have a generally V-shaped lower surface (when viewed in cross-section). In other embodiments, the lower surface of the perimeter beams may comprise a first region extending at a first angle and a second region extending at a different angle.

In other embodiments, the sloped lower surface comproses a single sloped lower surface.

In some embodiments, the perimeter beams are positioned on or above the screw piles. In some embodiments, the perimeter beams rest on the screw piles. In other embodiments, the perimeter beams rest on one or more supports or cradles positioned on the screw piles. In other embodiments, particularly in embodiments where the perimeter beams are formed on site, the perimeter beams may extend around an upper portion of the screw piles and extend above the top of the screw piles.

In other embodiments, the perimeter beams do not rest on the screw piles.

In one embodiment, the building system of the present invention comprises a plurality of precast perimeter beams being positioned above the screw piles. The perimeter beams may be supported on one or more supports or cradles positioned on the screw piles. The supports or cradles may have a support surface that is complementary in shape to a lower surface of the perimeter beams. In this embodiment, the perimeter beams may be tied to the supports or cradles, or tied to pile caps positioned on the upper part of the screw piles. Straps, rods or other ties may be used to tie the perimeter beams to the supports or cradles, or pile caps.

The supports or cradles may be provided with one or more shims or adjusting plates to alter or adjust the height of the perimeter beam relative to the top of the screw pile. The one or more shims or adjusting plates may comprise angled plates or V-shaped plates.

In one embodiment, the building system of the present invention provides a building slab system comprising a plurality of screw piles positioned in the ground and a slab constructed on and formed generally above the screw piles, the slab including a perimeter beam, characterised in that at least part of the perimeter beam has a sloped lower surface. The sloped lower surface may divert swelling soil away from the perimeter beam.

In one embodiment, the sloped lower surface diverts swelling soil towards an outer edge of the slab.

In another embodiment, the slab includes one or more void formers, and the sloped lower surface of the perimeter beam diverts swelling soil to a region occupied by the one or more void formers. The one or more void formers may comprise void formers that are designed to be crushed when subjected to pressure from swelling soils under the slab. For example, the one or more void formers may comprise void formers that are designed to crush in a predetermined mechanical fashion when subjected to hydraulic uplift pressures from heaving clay soils under the slab.

In some embodiments of the building slab system, the perimeter beams may rest on the screw piles. In other embodiments, other parts of the slab may rest on the screw piles.

In embodiments of the present invention, the perimeter beam is formed by setting out appropriate formwork, the void formers are positioned, appropriate reinforcement is laid and concrete poured so that it covers the void formers to a predetermined depth (to form a continuous upper concrete surface of the slab) and extends around the void formers to form the perimeter beam. Those skilled in the art will appreciate that the perimeter beam of the slab represents a region of increased concrete thickness.

It will also be appreciated that the slab may incorporate other beams. For example, additional beams may be formed in regions where additional load carrying capacity is required in the slab, such as regions of a slab under load bearing walls. Suitably, the additional beam or beams also have a sloped lower surface.

The beams of the slab comprise a sloped lower surface. In one embodiment, the beams of the slab comprise two sloped lower surfaces. For example, the beams may have a generally V-shaped lower surface (when viewed in cross-section).

The sloped lower surface of the beams may be formed by appropriately shaping the ground in the vicinity of the beams and pouring concrete on to the appropriately shaped ground. However, it is preferred that the sloped lower surface of the beams is formed by using an appropriate forming element. For example, if it is desired to produce beams that have a generally V-shaped lower surface, a generally V-shaped forming member may be placed in the vicinity of beams and concrete subsequently poured into the forming member to form the lower surface of the beams. The generally V-shaped forming member may comprise a generally V-shaped metal sheet or plastic sheet.

In some embodiments, the sloped lower surface of the beams may slope upwardly at an angle of least 20° to horizontal, preferably at least 22° to the horizontal, more preferably at about 30° to the horizontal. Higher angles may also be used, such as up to 45° or even higher, if determined to be desirable in particular soil types, for example as a result of a geotechnical engineering analysis. In this regard, the present inventor has found that clay has a natural separation of its electrostatically charged molecular bonds when an angular force is applied. When the angular force that is applied has a pitch of between 22° to 30° (depending upon the clay type and particulate type in the clay), the clay will naturally separate and cleave under very little applied pressure and therefore can be diverted sideways as the soil swells (indeed, the angle for cleaving of the clay without the weight of the slab and structure on the slab can be demonstrated by testing the clay to determine its natural angle of repose). As clay under the slab swells, a force is applied to the clay by virtue of the weight of the slab and the building constructed on the slab and this force, in conjunction with the sloped lower surface of the beams, can cleave and divert the swelling clay. In this manner, the swelling soil can be diverted away from the beams and move either externally to the slab or into voids or void formers positioned underneath the slab. In this manner, the swelling soil is effectively forced to swell upwardly into voids or upwardly and outwardly away from the slab. In either case, the massive hydraulic forces that would otherwise be applied by the swelling soil to the building slab are diverted sideways and externally to the slab and/or into voids or void formers within the slab so that the soil swells whilst applying only a relatively small pressure to the building slab. The present invention encompasses a sloping lower surface that has sufficient slope to divert the swelling soil or clay.

In some embodiments of the present invention, the sloped lower surface of the beams may slope upwardly at an angle of about 30° to horizontal. As mentioned above, the weight of the slab and the building constructed on the slab force the swelling clay or soil to be diverted sideways. Using an angle of 30° ensures that a reasonable safety factor is built into the sloping lower surface to ensure that satisfactory diversion of the swelling clay or soil takes place. It will be understood that angles of less than 30° may also result in diversion of the swelling clay or soil. In embodiments where the beams have a V-shaped lower surface, each sloped side of the V-shaped lower surface may slope upwardly at an angle of least 20° to horizontal, more preferably about 30° to horizontal. The building system of the first aspect of the present invention includes a plurality of screw piles. In some embodiments, each screw pile is fitted with a pile cap, such as the pile cap described in our Australian patent number 2007237161. This enables a structural slip joint to be formed between the pile cap and the slab.

In second aspect, the present invention provides a method for constructing a building slab comprising the steps of positioning a plurality of screw piles in the ground, and forming a concrete slab above the screw piles, the concrete slab having a perimeter beam, wherein at least part of the perimeter beam has a sloped lower surface. In some embodiments, the perimeter beam has two sloped lower surfaces, such as opposed sloping lower surfaces. The two sloped lower surfaces may comprise a V-shaped lower surface (when viewed in cross-section along the beam).

In some embodiments, the method of the present invention further comprises the step of placing pile caps on each of the screw piles prior to forming the concrete slab.

The method may further comprises the step of positioning slope formers for forming the sloped lower surfaces of the beam prior to forming the concrete slab. The slope formers may comprise V-shaped sheets or members. The V-shaped sheets may be positioned over one or more of the screw piles. For example, appropriate holes may be formed in the V-shaped sheets and the V-shaped sheets positioned by placing the holes over the screw piles and sliding the V-shaped sheets down along the shaft of the screw pile. Pile caps may then be placed onto the one or more screw piles that have the V-shaped sheets positioned thereon.

The method may further comprises the step of positioning one or more void formers prior to forming the concrete slab. The one or more void formers may comprise crushable void formers. In the finished slab, the one or more void formers may be positioned under the slab and interiorly of the perimeter beam.

In one embodiment of the method of the present invention, a plurality of screw piles are positioned in the ground, such as by screwing the screw piles into the desired position in accordance with an engineering design for the slab. The screw piles are either screwed in until their height above the ground surface is at the correct relative level (RL) or the screw piles are screwed into the ground until they reach the minimum required depth and the top of the screw piles may then be cut to the desired relative level. Laser leveling may be used to ensure that the top of the screw piles are at the correct level. Suitably, the top of all the screw piles are at the desired relative level. Pile caps may be positioned on top of the screw piles. Suitably, each screw pile is fitted with a pile cap. A relatively thin layer of sand or other particulate material may be used to fill between the screw piles. The slope formers are also positioned as required. The slope formers may rest on or be supported by the sand or other particulate material.

Once the sand or other particulate material has been levelled, the void formers are placed in position and a waterproof membrane, such as plastic sheeting is placed over the sand, void formers and the pile caps. Suitably, the plastic sheeting is positioned below the slope formers. This further ensures separation between the slab and the screw piles, in accordance with building codes. Appropriate form work is positioned, reinforcement material laid as required and the concrete is poured to form the concrete slab.

In a further aspect, the present invention provides a method for constructing a building comprising the steps of positioning a plurality of screw piles in the ground and placing one or more perimeter beams above the screw piles, wherein at least part of the perimeter beams has a sloped lower surface. In some embodiments, the perimeter beam has two sloped lower surfaces, such as opposed sloping lower surfaces. The two sloped lower surfaces may comprise a V-shaped lower surface (when viewed in cross-section along the beam).

In some embodiments, the method of the present invention further comprises the step of placing pile caps on each of the screw piles prior to placing the perimeter beams above the screw piles.

In a further aspect, the present invention provides a building having a perimeter beam, characterised in at least part of the perimeter beam has a sloped lower surface. The sloped lower surface may divert swelling soil away from the beam

In a further aspect, the present invention provides a concrete slab having a perimeter beam, characterised in that at least part of the perimeter beam has a sloped lower surface. The sloped lower surface may divert swelling soil away from the beam. In this aspect of the present invention, screw piles may or may not be used. The concrete slab may include one or more voids or void formers in the underside of the slab and the swelling soil may be diverted into those regions.

In all aspects of the present invention, substantially all of the perimeter beam has a sloped lower surface. In other embodiments, the slab includes a perimeter beam on the underside of the slab and one or more other beams on the underside of the slab and the beams have a sloped lower surface. The sloped lower surface of the beams is suitable constructed such that it rests on the ground when initially constructed.

Regions of the slab located away from any beams are suitably supported by one or more screw piles. The beams may also be constructed on or over one or more screw piles. The screw piles underneath the beams may be at a lower relative level than the screw piles that support the slab at regions located away from the beams.

In order to more clearly understand the benefits and advantages arising from the present invention, embodiments of the present invention will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side schematic view of a building slab in accordance with an embodiment of the present invention;

FIG. 2 shows an enlarged schematic side view of part of the building slab in accordance with an embodiment of the present invention;

FIG. 3 shows a cross sectional view of a beam of the building slab having a sloped lower surface (in this case, a V-shaped lower surface) and the cleaving and diversion of a swelling clay soil under the beam of the building slab;

FIG. 4 shows a perspective view from underneath a screw pile having a slope forming member positioned thereon;

FIG. 5 shows a perspective view from above of a screw pile having a slope forming member positioned thereon and a pile cap mounted on the screw pile;

FIG. 6 is a plan view of an embodiment of a void former suitable for use in the present invention;

FIG. 7 is an underneath view of the void former shown in FIG. 6;

FIG. 8 is a plan view of an arrangement of void formers that may be used in the construction of a building slab in accordance with an embodiment of the present invention;

FIG. 9 is an elevation view showing internal details of the void former shown in FIGS. 6 and 7;

FIGS. 10A to 10C show side views of crushing of a void former by swelling clay soil;

FIG. 11 is a plan view showing a steel up and boxing site plan of a building slab in accordance with an embodiment of the present invention

FIG. 12 is a schematic view showing a footing or foundation for a building using precast perimeter beams;

FIG. 13 is a cross sectional view of a precast perimeter beam suitable for use in an embodiment of the present invention;

FIG. 14 is a cross-sectional view of another precast perimeter beam suitable for use in an embodiment of the present invention;

FIG. 15 is a perspective view of a support or cradle suitable for use with the perimeter being shown in FIG. 14;

FIG. 16 is an end view of the support or cradle shown in FIG. 15;

FIG. 17 is a perspective view of a shim suitable for use with the cradle shown in FIG. 15;

FIG. 18 is an end view of another support cradle suitable for use with the perimeter beam shown in FIG. 13;

FIG. 19 is a schematic view showing use of the precast perimeter beam shown in FIG. 14 with the shim and cradle;

FIG. 20 is an end view of a screw pile having an alternative support attached to it;

FIG. 21 is an end view of another screw pile having a support attached to it; and

FIG. 22 shows a cross-sectional side view of an alternative embodiment of a bilding system in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The person skilled in the art will appreciate that the drawings have been provided for the purposes of illustrating preferred embodiments of the present invention. Therefore, it will be understood that the present invention should not be considered to be limited to the features as shown in the attached drawings.

FIG. 1 shows a side schematic view of a building slab system in accordance with an embodiment of the present invention. In FIG. 1, the building slab 10 comprises an upper concrete surface 12, 14. The slab 10 also includes a plurality of beams 16, 18, 20 (which may also be referred to as perimeter beams, thickening beams or footings). The plurality of beams strengthen the slab.

In order to form the beams, appropriate void formers 22, 24, 26, 28, 30 and 32 are positioned in required locations. Formwork is established and the concrete is poured over the void formers. The concrete fills the spaces between the void formers to form any interior beams and the spaces between the void formers and the formwork to form the perimeter beams. The concrete also extends above the void formers to form the upper surfaces 12, 14 of the slab.

The building slab system shown in FIG. 1 also includes a number of screw piles 34, 36, 38 and 40. The screw piles are screwed into the ground to a depth below the zone of influence (which is the zone at which changes in the soil properties are exhibited, for example, swelling and shrinkage due to changes in moisture content of the soil) so that he end of the screw piles extends into a stable zone. This acts to firmly root the screw piles in the stable zone. The screw piles may comprise twin blade piles as described in our international patent application number PCT/AU2008/001668, the entire contents of which are incorporated herein by cross reference.

As can be seen from FIG. 1, screw pile 36, which is located away from a beam, has its top at a higher level than the screw piles 34, 38 and 40, which are associated with beams. This difference in relative level for the screw piles accounts for the different (and lower) thickness of the slab at regions located away from the beams.

Each screw pile 34 to 40 is also equipped with a pile cap 42, 44, 46 and 48. The pile caps result in the formation of a structural slip joint between the concrete slab 10 and the screw piles. The pile cap 44 is positioned on top of screw pile 36 and the concrete slab is positioned above pile cap 44. In this manner, the slab rests on top of pile cap 44 and the screw pile 36 supports the lower surface of the slab. The pile caps 42, 46 and 48 that are located on screw piles positioned underneath the beams 16, 18 and 20 are partially encased in the concrete that forms the beams. However, any concrete that sets and forms a collar under these pile caps is only present in a small amount and should an upward force be exerted on the beams, that small concrete collar around the pile cap will readily fracture to thereby enable the structural slip joint to be established.

As shown in FIG. 1, the beams 16, 18 and 20 each have a sloped lower surface (when viewed in cross-section). In particular, the sloped lower surface of beam 16 comprises a first sloped region 50 and a second sloped region 52. Sloped regions 50 and 52 are arranged such that the beam has a generally V-shaped lower surface. Sloped regions 50 and 52 may extend at approximately 30° to horizontal.

A thin layer of sand 53 is positioned at the desired height and screed to be level. This layer of sand is suitably about 50 mm thick to comply with Australian standards. Appropriate slope forming members are then positioned over the respective screw piles 34, 38, 40 and are embedded into the thin layer of sand 53. The void formers 22 to 32 are then positioned in place. A waterproof membrane, in the form of a plastic sheet 54, is positioned over the slope forming members and the void formers. Appropriate reinforcing members are then properly positioned and the concrete is poured to form the slab 10.

As can be seen from FIG. 1, the lower surface of the beams rests on the ground when the slab is constructed. However, the regions of the slab located away from the beams are spaced above the ground level. The void formers act to space the lower surface of the slab in these regions above the ground surface.

FIG. 2 shows an expanded view of a part of a building slab system that is generally similar to the building slab system shown in FIG. 1. For convenience, like reference numerals will be used to describe like features. These features need not be described further. However, shown in further detail in FIG. 2 is the use of a slope former 60. Slope former 60 is in the form of a V-shaped sheet or member having a generally channel shape. The V-shaped member has generally vertical sidewalls 62, 64 and sloping lower walls or bottom 66, 68. Sloping wall 66 extends at an angle of about 30° to the horizontal, as does sloped wall 68. Sloping wall 66 and sloping wall 68 form an apex located approximately in the middle of the V-shaped member. The slope formers may be positioned so that the lower surface of substantially all of the ribs or beams of the concrete slab have the sloped lower surface.

Also shown in FIG. 2 is formwork 70, 72, which is held in place by formwork supports 74, 76. Reinforcing bars, denoted by reference numeral 78 are also shown more clearly in FIG. 2.

FIG. 3 shows a cross sectional view of a beam of the building slab having a sloped lower surface (in this case, a V-shaped lower surface) and the cleaving and diversion of a swelling clay soil under the beam of the building slab. In FIG. 3, the V-shaped former 66 forms part of the beam 16. As a clay soil that is located underneath beam 16 increases in moisture content, it swells or expands. In a normal slab, this would act to cause uplift of the concrete slab. However, as the beam 16 has sloped lower walls, as the clay expands and tries to move upwardly, the angle of the sloped lower walls causes cleaving of the clay (which occurs due to separation of molecular bonds in the clay) at the apex of the beam. The deadweight of the slab (and the building that has been constructed on the slab) forces the clay to cleave around the structure and to be diverted to either side of the structure. The arrows in FIG. 3 show how the clay/soil is diverted outwardly as it swells. The clay that is diverted to region 80 is diverted externally of the periphery of the building slab whilst the clay that is diverted to the region 82 is diverted into the region occupied by void former 22 (see FIGS. 1 and 2). The void former 22 is suitably designed so that it can be crushed by the clay that swells up into it, thereby providing room for pressure relief.

FIGS. 4 and 5 show perspective views from below and above, respectively, of a screw pile 34 having a slope former 60 positioned thereon prior to pouring the concrete. In this regard, the void former 60 comprises a V-shaped member that is suitably made from a pressed or folded metal sheet. The sheet may be formed with three creases 61, 63 and 65 to delineate the portions 62, 64, 66 and 68. In one example, the V-shaped member may be up to 6 m long. Each of the two V sweeps (66, 68) may be approximately 167 mm long and the side sections (62, 64) may be about 83 mm high.

In order to fit the V-shaped member to the screw pile 34, an appropriately sized hole is cut into the V-shaped member and the V-shaped member is slipped over the shaft of the screw pile 34. The bottom of the V-shaped member will come into contact with the ground or with a thin layer of sand positioned on the ground. This acts to support the V-shaped member at the appropriate height. A pile cap 42 (which may be identical to the pile cap described in our Australian patent number 2007237161) is then positioned over the top of the screw pile. This also assists in maintaining the V-shaped member in position.

In order to form the beams of the building slab, void formers are used (in conjunction with formwork in the case of peripheral beams). FIG. 6 shows a top view of a void former 22 suitable for use in the present invention. The void former 22 is suitably made from polystyrene. It comprises a top surface 90. Appropriate guide marks 92 may be provided on the top surface if it is desired to cut the void former to smaller usable units, to fit under odd sized corners or beamed areas when required.

FIG. 7 shows an underneath view of the void former 22 shown in FIG. 6. The void former 22 includes downwardly extending external walls 93, 94, 95, 96. The void former 22 also includes a number of downwardly extending projections, some of which are numbered at 97, 98, 99. These projections extend downwardly from a lower surface of the top 90 of the void former 22. These projections extend all the way to the bottom of the void former 22. In this manner, the projections provide support for the void former and enable workmen to walk across the top surface 90 of the void former 22 without falling through the top surface. However, due to the shape of the projections, as clay swells and is diverted into the region occupied by the void former 22, the clay acts to compress and crush the projections, thereby providing room for the swelling clay to move into without placing undue loads on the building slab. This will be described in more detail with reference to FIGS. 10A to 10C.

FIG. 8 shows a plan view of a cluster of void formers 22A, 22B, 22C and 22D positioned next to each other for use in forming a large void in the building slab. The void formers are placed in close abutment with each other to largely prevent poured concrete from entering between the void formers.

FIG. 9 shows a side elevation view of void former 22. In FIG. 9, internal details of the void former 22 are shown in dotted outline. It can be seen at the void former 22 includes sidewalls 93 and 95 as well as the internal projections 97, 98, 99. The roof portion 100 of the void former 22 includes arched regions 101 between the downwardly extending projections 97 and 98, etc. A plurality of hollows 102 are also defined.

FIG. 10A shows the void former 22 in position at construction. In FIG. 10B, the clay or soil has swelled, causing projections 98 and 99 to become crushed and occupy some of the volume of hollows 101. In this regard, the clay or soil moves upwardly and outwardly as it swells and the sideways component of the swelling movement (which is caused by the sloped lower surfaces of the beams) assists in crushing the projections 98 and 99. In FIG. 10 C, the clay has swollen even further, leading to even further crushing of the projections of the void former 22. Therefore, it can be seen that swelling of the clay into the region occupied by the void former (and recognizing that the swelling clay moves into that region by the action of the sloped lower surfaces of the beams of the building slab) results in crushing of the projections of the void former which minimises the amount of uplift force applied to the building slab of the swelling clay.

The present applicants have found that a void former 22 having a height of 200 mm can crush up to 170 mm before applying excessive pressure to the underside of the slab. It is anticipated that this amount of crushing should provide effective pressure relief for the swelling of clay soils in even extremely highly reactive soils.

Furthermore, if extremely high swelling does occur and the swelling soil crushes the void formers to full extent, further swelling of the soil will cause lifting of the building slab. However, as a structural slip joint exist between the building slab and the screw piles, the slab can move upwardly. When the soil dries and shrinks, the slab (and pile caps) will move downwardly until the pile caps again seat or rest on the top of the screw piles.

FIG. 11 shows a steel up and boxing site plan that may be used in embodiments of the present invention. In FIG. 11, the peripheral beams of the slab are shown by reference numeral 110. These peripheral beams 110 each have the cross-sectional configuration as shown in FIGS. 1 and 2

In order to construct the building slab system of the present invention, the site is surveyed and/or external corner pegs are positioned in the ground. The screw piles and pile caps are laser level installed to the required locations and relative levels (RLs). Plumbing services are installed after screw pile installation. A thin layer of bedding sand is then placed over the pad area. The void formers are positioned on the sand, pushed together as required with the piles pushed through and the top of the pile caps sitting flush with the top of the void formers (where required). A plastic membrane is placed on the piles and void formers. The slope formers, typically in the form of V-shaped sheets, are fitted into place into the bedding sand and against the void formers. The V-shaped sheets have holes cut out to allow the screw piles to penetrate and the pile caps are refitted onto the piles inside the V-shaped sheet. Steel mesh and bars, etc are placed and chaired. External form boards and shutters are set for the perimeter beams. Trench mesh is placed, chaired and tied in with the balance of steel needs. The concrete is poured and vibrated into place.

The slab can then be stripped and finished.

Embodiments of the present invention provide a number of advantageous features. For example, the void formers (or pods) provide an under slab formwork pod that is designed to crush in a predetermined mechanical fashion when subjected to hydraulic uplift pressures from heaving clay soils under the slab. The pod uses bridge design engineering principles to main strength during construction from pressures exerted above by trades people, steel reinforcing, concrete and concrete pumps. The pod distributes the top load over the area of the pod by a group of networked triangular, diamond shaped and arched crushable hollow sections. They are designed to crush from the base up, in a predetermined direction and height. The pods may be placed under the entire slab area.

The pile cap is designed to function as a controlled structural strip joint, sliding up and down on the top of a steel pile with the slab during periods of heave or shrink in reactive clay soil. The top section of the pile cap creates an enlarged support area between the slab and the pile. The pile cap also isolates the slab from the supporting steel pile, enabling the slab design to remain classed as a floating raft slab in accordance with Australian standards for residential raft slab designs (in particular, AS2870) whilst simultaneously being designed as a fully suspended concrete slab in accordance with AS3600 (Australian standard for commercial grade suspended slab structures). Effectively, the slab is now a fully suspended raft slab, which is a first ever for concrete slab designs.

Construction of the slab utilises slope formers to form sloped lower surfaces on the beams of the slab. It forms the base of all perimeter and internal beam areas that are in contact with the ground. Creating a sloped lower surface or a V bottom to all beams enables clay soils to cleave apart to the outside of the slab perimeter or into the internal crushable pods when swelling of the soil against the underside of the sloped lower surface or V bottom occurs, instead of lifting the beams and slab structure.

In embodiments of the slab design, the slab is constructed to a fixed position RL (Relative Level). The screw piles are installed with a laser level to establish the fixed RL height, as determined by the engineer or builder. Piles installed to all beam areas may be installed, for example, 100 mm above underside of slab RL. All internal piles that are located away from beams may be installed, for example, 200 mm above underside of slab RL. Pile caps in the beam areas are designed to be installed within the concrete V beam. Internal piles are designed to support the underside of the main slab areas. Pods (void formers) are designed to be pushed together to act as sacrificial form work under the main slab areas, flush with the top of the pile caps that are on the internal piles. V form sheets (slope formers) are placed on the ground under all internal and perimeter beam areas. Holes are cut in the base of the V sheets to allow the beam area piles to penetrate into the beam, then a pile cap is fitted to the top of the pile. V Sheets are miter trimmed at corner junctions to maintain the V bottom shape. External form boards and the internal pods hold the V form sheets in place prior to steel and concrete placement. A waterproof membrane is placed over the pods and the concrete is subsequently poured. The waterproof membrane may be positioned underneath the V sheets.

In some embodiments, the void formers or pods may be manufactured using a special decomposition mix of polystyrene. Such a decomposition mix ensures long-term environmental effects are minimised.

FIG. 12 shows a schematic view of details of a footing or foundation for use in a building. The footing or foundation shown in FIG. 12 includes a plurality of screw piles 200 that support a plurality of perimeter beams 202, 204, 206, 208, 210, 212 and 214. In constructing the footing or foundation shown in FIG. 12, the plurality of screw piles 200 are screwed into the ground and set at the correct relative level (RL). The plurality of perimeter beams are then positioned on top of the screw piles. The perimeter beams are suitably precast reinforced concrete beams that are manufactured in a factory and transported to site. The perimeter beams may be tied or otherwise joined to the screw piles. Alternatively, the perimeter beams may simply rest on the screw piles. The screw piles may be fitted with pile caps.

The perimeter beams shown in FIG. 12 are entirely conventional and they have a generally flat lower surface. Indeed, the perimeter beams shown in FIG. 12 have a generally rectangular cross section.

The footing or foundation shown in FIG. 12 may be arranged such that the perimeter beams are positioned above the ground surface, positioned on the surface of the ground or even positioned in the trenches dug into the ground.

FIGS. 13 and 14 show end views or cross sectional views of perimeter beams suitable for use in embodiments of the present invention. In FIG. 13, the perimeter beam 220 includes side surfaces 222, 224 and a top surface 226. The lower surface, generally indicated by reference numeral 228, includes a first angled surface 230 and a second angled surface 232. Angled surface 230, 232 form a generally V-shaped lower surface.

FIG. 14 shows a perimeter beam 240 that includes side surfaces 242, 244 and a top surface 246. The lower surface, generally indicated by reference numeral 248, includes a first angled surface 250 that extends as a first, relatively steep angle and a second angled surface 252 that extends at a different, less steep angle than surface 250. Similarly, the lower surface 248 further includes another angled surface 254 that extends at a first, relatively steep angle and a second angled surface 256 that extends at a different, less steep angle than surface 254.

The perimeter beams 220 and 240 shown in FIGS. 13 and 14 allow a swelling soil to be diverted sideways and away from the beams, in a manner similar to that shown with reference to FIGS. 1 to 11.

Due to the lower surface of the perimeter beams shown in FIGS. 13 and 14 having angled sections (and indeed, due to them being generally V-shaped), it is likely that special supports or cradles will be required in order to support the perimeter beams in the correct orientation. One such example is shown in FIGS. 15 and 16.

The cradle shown in FIGS. 15 and 16 is suitable for supporting the perimeter beam 240 shown in FIG. 14. In particular, the support or cradle 260 shown in FIG. 15 comprises a cradle body 262 that is generally cylindrical in cross section. It will be understood that a myriad of different cross sectional shapes for the cradle body may be used. The cradle body may be made from steel, concrete, heavy-duty plastic or the like. Generally, the cradle body should be substantially incompressible.

The cradle body 262 also includes a recess that is of complementary shape to the shape of the lower surface 248 of perimeter beam 240. In particular, the recess includes a lower section 264 that includes side walls that extend at the same angle as regions 250, 254 of beam 240. The recess includes outer regions 266, 268, that extend at essentially the same angle as regions 252, 256 of perimeter beam 240. Thus, the perimeter beam 240 can neatly fit into the recess of the cradle 260. As a result, the cradle 260 supports the perimeter beam 240 in a generally upright configuration.

Also shown in FIGS. 15 and 16 are a pile 270 that is positioned on top of a screw pile 272 (see FIG. 16). The pile cap 270 may be as described in our Australian patent number 2007237161. The cradle 260 may rest on the pile cap 270. Alternatively, the cradle 260 may be fixed to the pile cap 270.

FIG. 17 shows a shim 280 that comprises an angled plate. Shim 280 may be formed by pressing a single sheet of metal. Alternatively, two separate metal plates may be joined together to form the shim 280. The shim 280 may be placed in the recess of the cradle 260 to adjust the height of the perimeter beam.

FIG. 18 shows another cradle 290 that has a cradle body 292 and a recess having a first angled wall 294 and a second angled wall 296. Angled walls 294, 296 correspond to the lower sections 230, 232 of the perimeter beam 220 shown in FIG. 13.

FIG. 20 shows an arrangement of the perimeter beam 240, the shim 280, the cradle 260 and the pile cap 270 in position for assembly. Advantageously, a strap or tie 300 may be used to tie the perimeter beam to either the cradle 260, the pile cap 270 or to the screw pile. The strap or tie 300 may comprise a metal strap having holes 302 allow fasteners, such as bolts, screws or the like to be used to connect the strap to be perimeter beam and to the cradle, the pile cap the screw pile.

FIG. 21 shows a side view of a screw pile suitable for use in embodiments of the present invention. The screw pile 320 includes a pile shaft 322 having opposed twin blades 224, 326. Lugs 328 are mounted to the top of the pile in order to enable the pile to be rotated by drive apparatus so that it can be screwed into the ground. The pile 320 may be generally similar to the twin blade screw pile shown in our international patent application number 2007237161, the entire contents of which are herein incorporated by cross reference. The screw pile 320 may be fitted with a nut 330 that receives a threaded bar 332. Threaded bar 332 may hold a support 334 that comprises a base plate 336 having angled support walls 338, 340. Support walls 338, 340 are angled so that they support the lower surface of the perimeter beam. The support walls 338, 340 may be welded to the base plate 336. Weld metal, as shown at 342, may be used to hold the support walls 338, 340 in the correct orientation. One or more other reinforcing members (not shown) may also be used to hold the support walls 338, 340 in position.

In use of the screw pile 320 shown in FIG. 20, the screw pile is initially screwed into the ground. The threaded bar 332 is then screwed into the nut 330 until the base plate 336 is at the correct height. The perimeter beam may then be positioned on the support 334.

In an alternative embodiment, the base plate and support may be permanently affixed to the screw pile, for example by having the base plate and support mounted to a sleeve that is positioned over the top part of the screw pile and then welded to the screw pile, or by simply welding the base plate to the screw pile.

FIG. 21 shows an alternative screw pile 360 also suitable for use in the present invention. The screw pile 360 includes a number of features that are common with the screw pile 320 and, for convenient and brevity of description, those features will be denoted by the same reference numerals as used in FIG. 20. These features need not be described further. Where the screw pile 360 of FIG. 21 differs from the screw pile 320 shown in FIG. 20 is that screw pile 360 also includes lateral projections 362, 364, 366. These lateral projections are in the form of fins that extend outwardly from the screw pile. The fins are mounted to a cylindrical housing 368 that is positioned around the shaft of the screw pile (the shaft of the screw pile may have a recessed diameter in the region of cylindrical housing 368 for neatness of appearance). The cylindrical housing 368 can rotate relative to the shaft of the screw pile in 360.

When the screw pile 360 is driven into the ground, the fins 362, 364, and 366 will eventually engage with the ground as the screw pile digs deeper into the ground. As the cylindrical housing 368 can rotate relative to the shaft of the screw pile 360, as rotation of the screw pile continues to cause the screw pile to dig into the ground, the pressure of the earth on the fins prevents rotation of the fins. Therefore, as the screw pile is driven into the ground, the fins are also driven down into the ground. The fins act to compress the earth and set up an increased reaction force in the earth. This effectively increases the “hold” of the screw pile in the ground, thereby adding to the stability of the screw pile in the ground. The present inventor has found that the addition of the fins increases the resistance of the screw pile to movement upon the application of a lateral force to the screw pile. The present inventor also believe that the particular configuration of the fins, which acts to force the earth away from the fins (and away from the screw pile) as the screw pile is driven into the ground increases the region of compressed earth surrounding the screw pile when the screw pile is in its final position. This results in a firm hold of the screw pile in the ground.

The screw pile as shown in FIG. 21 forms another aspect of the present invention.

As also shown in FIGS. 20 and 21, the screw piles 320 and 360 may be made from a lower section 350 and an upper section 352, with the lower section and the upper section being bolted together using brackets 354 and bolts 356. The lower section 350 may be provided with driving lugs 358.

FIG. 22 shows a cross sectional view of a building system in accordance with yet another embodiment of the present invention. The building system comprises a screw pile 400 having a pile cap 402 mounted thereon. A building slab 404, such as a concrete slab, rests on the pile cap 402. The building slab 404 includes a drop edge beam 406. The drop edge beam 406 is suitably integrally formed with the slab 404. As can be seen from FIG. 22, the drop edge beam 406 extends below the upper end of the screw pile 400.

The drop edge being 460 includes a single sloped lower surface 408. The sloped lower surface 408 may extend at an angle of, for example, about 45°. If the soil that is positioned below the sloped surface 408 expands, the shape of the lower part of the drop edge beam 406 acts like a knife or blade to divert the swelling soil away from the beam.

In the embodiment shown in FIG. 22, the sloped lower surface 408 of the drop edge beam 406 diverts swelling soil towards the outside of the slab 404. It will be appreciated that the sloped lower surface could also be arranged so that it diverts the swelling soil towards the inside part of the slab. In this instance, the swelling soil will be diverted into a void that would normally be present between the ground surface and the underside of the slab 404.

In the embodiment shown in FIG. 22, the drop edge beam 406 may be a relatively narrow beam, such as a beam that is from 100 to 120 mm wide. The distance from the lowermost part of the drop edge beam 406 to the top of the slab may be from about 300 mm to about 450 mm. The slab itself may have a thickness of from 100 mm to 200 mm. Of course, the slab and drop edge beam will need to be engineered to meet the strength requirements for the particular building and the geotechnical properties of the surrounding soil.

The building system of the present invention may comprise a fully suspended structure on which the building system is suspended on screw piles.

Those skilled in the art will appreciate that the present invention may be susceptible variations and modifications other than those specifically described. It will be understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope. 

1. A building system, characterised in that the building system includes a plurality of screw piles positioned in the ground and one or more perimeter beams, wherein the perimeter beams have a lower surface adapted to cleave swelling soil.
 2. A building system as claimed in claim 1 wherein the lower surface of the perimeter beam is further adapted to divert swelling soil away from the perimeter beam.
 3. A building system as claimed in claim 1 wherein the lower surface of the perimeter beams comprises a single sloped lower surface or two sloped lower surfaces.
 4. A building system as claimed in claim 3 wherein at least one of the perimeter beams has a generally V-shaped lower surface when viewed in cross-section or the lower surface of at least one of the perimeter beams comprises a first region extending at a first angle and a second region extending at a different angle.
 5. A building system as claimed in claim 1 wherein the perimeter beams are positioned on or above the screw piles, or the perimeter beams rest on the screw piles, or the perimeter beams rest on one or more supports or cradles positioned on the screw piles, or the perimeter beams extend around an upper portion of the screw piles and extend above the top of the screw piles.
 6. A building system as claimed in claim 1 comprising a plurality of precast perimeter beams being positioned above the screw piles.
 7. A building system as claimed in claim 6 wherein the perimeter beams are supported on one or more supports or cradles positioned on the screw piles and the supports or cradles have a support surface that is complementary in shape to a lower surface of the perimeter beams.
 8. A building system as claimed in claim 7 wherein the perimeter beams are tied to the supports or cradles.
 9. A building system as claimed in claim 6 wherein the perimeter beams are tied to pile caps positioned on the upper part of the screw piles.
 10. A building system as claimed in claim 8 wherein the supports or cradles are provided with one or more shims or adjusting plates to alter or adjust the height of the perimeter beam relative to the top of the screw pile.
 11. A building system as claimed in claim 10 wherein the one or more shims or adjusting plates comprise angled plates or V-shaped plates.
 12. A building system as claimed in claim 1 wherein the building system comprises a plurality of screw piles positioned in the ground and a slab constructed on and formed generally above the screw piles, the slab including a perimeter beam, characterised in that at least part of the perimeter beam has a sloped lower surface.
 13. A building system as claimed in claim 12 wherein the slab includes one or more void formers, and the sloped lower surface of the perimeter beam diverts swelling soil to a region occupied by the one or more void formers.
 14. A building system as claimed in claim 13 wherein the one or more void formers comprise void formers that are designed to be crushed when subjected to pressure from swelling soils under the slab.
 15. A building system as claimed in claim 14 wherein the one or more void formers comprise void formers that are designed to crush in a predetermined direction and height when subjected to hydraulic uplift pressures from heaving clay soils under the slab.
 16. A building system as claimed in claim 15 wherein the perimeter beam is formed by setting out appropriate formwork, positioning the void formers, laying reinforcement and pouring concrete so that it covers the void formers to a predetermined depth to form a continuous upper concrete surface of the slab) and extend around the void formers to form the perimeter beam.
 17. A building system as claimed in claim 16 wherein the slab includes other beams.
 18. A building system as claimed in claim 17 wherein the other beam or beams also have a sloped lower surface.
 19. A building system as claimed in claim 12 wherein the sloped lower surface of the beams is formed by appropriately shaping the ground in the vicinity of the beams and pouring concrete on to the appropriately shaped ground, or by using a appropriate forming element.
 20. A building system as claimed in claim 19 wherein the beam has a generally V-shaped lower surface and a generally V-shaped forming is placed in the vicinity of beams and concrete subsequently poured into the forming member to form the lower surface of the beams.
 21. A building system as claimed in claim 20 wherein the generally V-shaped forming member comprises a generally V-shaped metal sheet or plastic sheet.
 22. A building system as claimed in claim 3 wherein the sloped lower surface of the beams slopes upwardly at an angle of between at least 20° to 45°.
 23. A building system as claimed in claim 1 wherein each screw pile is fitted with a pile cap such that a structural slip joint is formed between the pile cap and the slab.
 24. A method for constructing a building slab comprising the steps of positioning a plurality of screw piles in the ground, and forming a concrete slab above the screw piles, the concrete slab having a perimeter beam, wherein at least part of the perimeter beam's lower surface-is adapted to cleave swelling soil.
 25. A method as claimed in claim 24 wherein the perimeter beam has two sloped lower surfaces.
 26. A method as claimed in claim 25 wherein the two sloped lower surfaces comprise a V-shaped lower surface when viewed in cross-section along the beam.
 27. A method as claimed in claim 26 further comprising the step of placing pile caps on each of the screw piles prior to forming the concrete slab.
 28. A method as claimed in claim 27 further comprising the step of positioning slope formers for forming the sloped lower surfaces of the beam prior to forming the concrete slab.
 29. A method as claimed in claim 28 wherein the slope formers comprise V-shaped sheets or members.
 30. A method as claimed in claim 29 wherein the slope formers are positioned over one or more of the screw piles.
 31. A method as claimed in claim 30 wherein holes are formed in the slope formers and the slope formers are positioned by placing the holes over the screw piles and sliding the slope formers down along the shaft of the screw pile.
 32. A method as claimed in claim 31 wherein pile caps may then be placed onto the one or more screw piles that have the slope formers positioned thereon.
 33. A method as claimed in claim 32 further comprising the step of positioning one or more void formers prior to forming the concrete slab.
 34. A method as claimed in claim 33 wherein the one or more void formers comprise crushable void formers.
 35. A method as claimed in claim 34 wherein, in the finished slab, the one or more void formers are positioned under the slab and interiorly of the perimeter beam. 36-43. (canceled) 